memstick: mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

This patch fixes the following warnings:

drivers/memstick/host/jmb38x_ms.c: In function ‘jmb38x_ms_write_data’:
drivers/memstick/host/jmb38x_ms.c:261:15: warning: this statement may fall through [-Wimplicit-fallthrough=]
   host->io_pos++;
   ~~~~~~~~~~~~^~
drivers/memstick/host/jmb38x_ms.c:262:2: note: here
  case 2:
  ^~~~
drivers/memstick/host/jmb38x_ms.c:264:15: warning: this statement may fall through [-Wimplicit-fallthrough=]
   host->io_pos++;
   ~~~~~~~~~~~~^~
drivers/memstick/host/jmb38x_ms.c:265:2: note: here
  case 1:
  ^~~~
drivers/memstick/host/tifm_ms.c: In function ‘tifm_ms_write_data’:
drivers/memstick/host/tifm_ms.c:168:15: warning: this statement may fall through [-Wimplicit-fallthrough=]
   host->io_pos++;
   ~~~~~~~~~~~~^~
drivers/memstick/host/tifm_ms.c:169:2: note: here
  case 2:
  ^~~~
drivers/memstick/host/tifm_ms.c:171:15: warning: this statement may fall through [-Wimplicit-fallthrough=]
   host->io_pos++;
   ~~~~~~~~~~~~^~
drivers/memstick/host/tifm_ms.c:172:2: note: here
  case 1:
  ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>